The conversion of arachidonic acid to leukotrienes via the 5-lipoxygenase pathway is a significant pathway in the metabolism of this polyenoic fatty acid in certain mammalian tissues. Two groups of biologically active leukotrienes have been isolated. They are slow reacting substances (leukotrienes C4 and D4) and 5(S), 12 (R)-6, 8, 10, 14-eicosatetraenoic acid (leukotriene B4). Leukotriene C4 and leukotriene D4 elicit responses in experimental animal similar to immediate hypersensitivity reactions (known to be mediated by slow reacting substances). Leukotriene B4 is the most potent chemotactic agent derived from polyenoic fatty acids via lipoxygenase reactions. The formation of leukotrienes B4, C4, and D4 is linked to lipoxygerase metabolism of arachidonic acid via leukotriene A4, a liable 5, 6-epoxy eicosanoid. The majority of leukotriene A4 is leukocytes is converted to leukotriene B4 with only minute quantities of leukotrienes C4 and D4 being formed. The proposed studies include the determination of the metabolic fate of leukotriene B4 in vivo for elucidation of the structure of its characteristic metabolites (circulating and urinary). These studies are a prerequisite for the development of methods for measurement of levels of characteristic leukotriene B4 metabolites in body fluids as an index of in vivo production of leukotrienes.